This invention relates to spread-spectrum communications, and more particularly to a coding technique for a large area spread-spectrum CDMA system.
The growing popularity of personal communication services coupled with the scarcity of radio bandwidth resources has resulted in the ever-increasing demand for higher spectral efficiency in wireless communications. Spectral efficiency refers to the maximum number of subscribers that can be supported in a cell or sector under a given bandwidth allocation and transmission rate requirement. The unit of spectral efficiency is the total transmission rate per unit bandwidth within a given cell or sector. Obviously, the better the spectral efficiency is, the higher the system capacity will be.
Traditional wireless Multiple Access Control (MAC) systems, such as a Frequency Division Multiple Access (FDMA) system, a Time Division Multiple Access (TDMA) system, result in system capacity that is limited by the time-bandwidth (TB) product. It is impossible to increase the number of supportable subscribers under these MAC schemes. For example, assume that the basic transmission rate of a subscriber is 1/T samples per second and the allocated bandwidth is B Hz. Then, the time-bandwidth product is BT, which is the maximum number of supportable subscribers. It is impossible to support more than BT subscribers in FDMA and TDMA systems.
The situation is completely different under a Code Division Multiple Access (CDMA) system where the system capacity only depends on the Signal-to-Interference Ratio (SIR). Increasing the number of subscriber reduces the SIR, thus lowering the transmission rate. However, a subscriber will not be denied radio resource allocation. In other words, unlike FDMA and TDMA systems, a CDMA system does not have a hard upper bound (i.e. BT) on the number of supportable subscribers.
The capacity of a CDMA system depends on the interference level. As such, the ability to accurately control the interference level is critical to the performance and the successful operation of a CDMA system. There are four sources of interference in a CDMA system: The first type of interference, or noise, comes from various sources in the local environment, which cannot be control by the wireless communication system. The only way to alleviate noise interference is to use a low noise amplifier. The second type of interference is Inter-Symbol-Interference (ISI). The third type of interference is Multiple Access Interference (MAI) that is originated from other subscribers in the same cell. The forth type of interference is Adjacent Channel or Cell Interference (ACI) that is originated from other subscribers in the neighboring channel or cell. It is possible to reduce or eliminate ISI, MAI, and ACI by using high performance codes.
In a CDMA system, each subscriber has his/her own unique identification code. In addition, the subscribers"" spread-spectrum codes are orthogonal to each other. The orthogonality requirement is common to all multiple access schemes. If the communication channel is an ideal linear time and frequency non-dispersion system, and the system has high degree of synchronization, then the subscribers will stay orthogonal to each other. In reality, the communication channel is not ideal, and it is very difficult to achieve tight synchronization for communication channels with time and frequency dispersion. As a result, the ability to achieve orthogonality in a non-ideal communication channel with time and frequency dispersion is critical to the successful operation of CDMA systems.
It is commonly known that a mobile communication channel is a typical random time varying channel, with random frequency dispersion, due to Doppler shift effect, and random time dispersion, due to multi-path transmission effect. Random frequency dispersion results in the degradation in time selectivity of the received signal with unexpected fluctuation of the reception power level. Random time dispersion results in the degradation in frequency selectivity, which results in the unexpected variation in the reception level within each frequency component. This degradation results in reduced system performance and significantly lowers the system capacity. In particular, because of the time dispersion of the transmission channel, as a result of multi-path transmission, different signal paths do not arrive at the receiver at the same time. This results in the overlapping of neighboring symbols of the same subscriber and causes Inter Symbol Interference (ISI). On the other hand, the time dispersion of the channel worsens the multiple access interference. When the relative delay of signals of different subscribers are zero, any orthogonal code can achieve orthogonality. However, it is very hard to maintain orthogonality if the relative delay of signals of subscribers is not zero.
In order to reduce ISI, the auto-correlation of each subscriber""s access codes must be an ideal impulse function that has all energy at the origin, nowhere else. To reduce the MAI, the cross-correlations between multiple access codes of different subscribers must be zero for any relative delay. In the terms of orthogonality, each access code must be orthogonal to itself with non-zero time delay. The access codes must be orthogonal to each other for any relative delay, including zero delay.
For simplicity, the value of an auto-correlation function at the origin is called the main lobe and the values of auto-correlations and cross-correlations at other points are called side lobes. The correlation functions of ideal multiple access codes should have zero side lobes everywhere. Unfortunately, it is proved in Welch theory that there does not exist any ideal multiple access codes in the field of finite elements and even in field of complex numbers. The claim that ideal multiple access codes do not exist, is called the Welch bound. Especially, the side lobes of auto-correlation function and the side lobes of cross-correlation function are contradicted to each other; as side lobes of one correlation function become small, the side lobes of the other correlation function become big. Furthermore, NASA had done brute force searching, by using a computer, to search for all ideal codes. However, there has not been a breakthrough. Since then, not much research work has been done on the search of the ideal multiple codes.
NASA searched for the good access codes in the Group codes and the Welch bound in the sub-fields of complex numbers. Beyond the field of complex numbers, the ideal codes could exist. For example, B. P. Schweitzer found an approach to form ideal codes in his Ph.D thesis on xe2x80x9cGeneralized complementary code setsxe2x80x9d in 1971. Later, Leppanen and Pentti (Nokia Telecommunication) extended Dr. Schweitser""s results in the mixed TDMA and CDMA system. They broke the Welch bound in the high dimensional space. However, the utilization of frequency is very low and thus there is no practical value. There has not been any application of their invention in nearly 30 years. According to their invention, in a system of N multiple access codes, there requires at least N2 basic codes. Each basic code has length at least N chips. That means it needs N3 chips to support N addresses. For example, when N=128, with 16 QAM modulation, the coded spectral efficiency is only log2 16xc3x97128/1283=2.441xc3x9710xe2x88x924 bits/Hz. The more access codes, the lower the utilization of the spectral efficiency. This coding methodology reminds us that ideal multiple access codes can be achieved via complementary code sets. We should, however, avoid that the code length grows too fast with the required number of multiple access codes.
In addition, with technique of two-way synchronization, the relative time delay within each access code or between each other in a random time varying channel will not be greater than the maximum time dispersion of the channel plus the maximum timing error. Assuming that value is xcex94 second, so long as their correlation functions do not have any side lobes in a time interval (xe2x88x92xcex94, xcex94), there are no MAI and ISI between the access codes. The time interval that possesses the above property is called xe2x80x9czero correlation windowxe2x80x9d. It is obvious that the corresponding CDMA system will be ideal when the xe2x80x9czero correlation windowxe2x80x9d size is wider than the maximum time dispersion deviation of the channel, i.e. the time delays among multi-paths of the signal, plus the maximum timing error. At the same time, it is also true that the near-far effects are no longer effective. The well-known near-far effects is created by the overlapping of the side lobe of a signal source that is close to the base station receiver and the main lobe of a signal source that is far away from the base station receiver. The side lobe over-kills the main lobe, which causes high interference. The accurate, complicated and fast power control mechanism has to been used to overcome the near-far effects so that the energy of signals must be basically the same at the base station receiver. However, within the xe2x80x9czero correlation windowxe2x80x9d of the multiple access codes, there are no side lobes in the auto-correlation functions and cross-correlation functions under the working condition. The near-far effects no longer exist in the system. The complicated and fast power control mechanism will become less important and optional.
Therefore, the distinction between different CDMA systems lies mainly in the selected multiple access codes, i.e. in a good system, ISI and MAI must both be small, otherwise they must be larger.
Existing CDMA systems have either very low efficiency or have very short communications distance for example about several hundred meters or do nothing to MAI and ISI and then all that can be done is to alleviate them by using relatively good multiple access codes.
A general object of the invention is a CDMA spread-spectrum system for a large area synchronous communications system or a large area asynchronous communications system.
An object of the present invention is to provide a new coding method for use with a spread-spectrum transmitter and receiver to create a series of spread-spectrum multiple access codes that have the xe2x80x9cZero Correlation Windowxe2x80x9d in their auto-correlation functions and cross-correlation functions. Due to the creation of the xe2x80x9czero correlation windowxe2x80x9d, the fatal near-far effects in traditional CDMA radio communications is solved. The Multiple Access Interference (MAI) and the Inter-Symbol Interference (ISI) is eliminated. A high RF capacity radio system could be thus created based on the invention.
According to the present invention, as embodied and broadly described herein, a transmitter and a receiver are provided which use a set of Large-Area Code-Division-Multiple-Access (LA-CDMA) codes. The LA-CDMA codes are generated from the steps of generating a plurality of pluses, and generating a plurality of codewords.
The plurality of pluses has a plurality of intervals between each of the pulses in the plurality of pulses, respectively. Each pulse of the plurality of pulses has an amplitude and a polarity. Each interval of the plurality of intervals is unequal to other intervals of the plurality of intervals. Only one interval of the plurality of intervals is an odd number larger than a value of a smallest interval of the plurality of intervals. No value of any interval in the plurality of intervals is a sum of any two or more values of two or more intervals, respectively, in the plurality of intervals.
Each codeword in the plurality of codewords is different from other codewords in the plurality of codewords, by assigning, for each respective codeword in the plurality of codewords, a polarity to each pulse in the plurality of pulses. The plurality of codewords are either orthogonal, or bi-orthogonal, or trans-orthogonal. The codewords are generated such that a cross-correlation between any two codewords in the plurality of codewords has side-lobes with any of the values of zero, plus amplitude squared, and minus amplitude squared. Any codeword in the plurality of codewords has a zero correlation window in an auto-correlation function and the cross-correlation function with a magnitude equal to the amplitude squared and with a width equal to two times the smallest interval of the plurality of intervals.
Additional objects and advantages of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.